Updating Mail Restriction on General Tab from Interests and Vice Versa

At the Royal Exchange, we use the General Tab (mail restriction, phone restriction, emarket restriction) for the Box Office to update Data Protection, and the Interests Tab for the Website to update data protection.  We want Box Office to be able to make their DP updates on the Gen Tab in order to reduce the number of clicks.

Although the data is held differently we've designed each to have the same information stored.  Now the challenge is to write an update script - over-write General Tab restrictions with Interests and vice versa.

For new records that's easy - the 'complete' data protection can update the 'blank' one and vice versa.  For existing records where one is blank the same applies.

But for existing records where both have information stored we need to know which was updated most recently.  What date fields can we use for that?  We need a specific date field on the General Tab that reflects changes to mail restriction, phone restriction and emarket restriction only (not changes to the rest of the constituent record).

We can use the Create and Update date on Interests - but when one is 'deselected' the dates disappear which makes that tricky.  If anyone has solved that one, would be great to hear from you.

Is there a date updated field specific only to the mail/email/phone restrictions on the general tab?

Thanks

Mary